Optical semiconductor device and method of fabricating the same

ABSTRACT

An optical semiconductor device includes an optical semiconductor element, a semiconductor region, and a buried layer. The optical semiconductor element is formed on a semiconductor substrate. The semiconductor region opposes the optical semiconductor element and essentially surrounds the optical semiconductor element to form walls. The buried layer is arranged between the walls of the semiconductor region and the optical semiconductor element and formed by vapor phase epitaxy. In this optical semiconductor device, a distance between the wall of the semiconductor region and a side wall of the optical semiconductor element is larger in a portion in which the growth rate of the vapor phase epitaxy in a horizontal direction from the side wall of the optical semiconductor element and the wall of the semiconductor region is higher.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an optical semiconductor deviceand a method of fabricating the same and, more particularly, to anoptical semiconductor device having a semi-insulating buriedheterostructure wherein an optical integrated circuit consisting of aplurality of waveguides or an optical integrated circuit obtained byadding an electronic device thereto is formed by using a high resistivesemi-insulating semiconductor, the optical semiconductor device beingrepresented by an optical cross switch such as a side-light injectiontype bistable laser or a directional coupler, and a method offabricating the same.

[0002] GaInAsp/InP semiconductor lasers have a basic arrangement inwhich a layer structure near a light emission region is a doubleheterostructure consisting of a Ga_(1−x)In_(x)As_(y)P_(1−y) active layerabout 100 nm thick and p- and n-type InP layers (cladding layers). Thesecladding layers vertically sandwich the active layer and have a largerband gap than that of the active layer.

[0003] Due to effective carrier confinement by this doubleheterostructure, in th active layer of the double heterostructure , itis possible to form excitation carriers at a high density (up to 10¹⁸cm⁻³) upon energization at a relatively low current density (1 to 10kA/cm²).

[0004] Also, the refractive index of a cladding layer having a largeforbidden band is generally smaller than that of an active layer. Thedouble heterostructure uses this refractive index difference to form anoptical waveguide in the direction of thickness, confining laser lightin the vicinity of the active layer.

[0005] A semiconductor laser having this double heterostructure with theabove properties can continuously oscillate at room temperature, so thedouble heterostructure is used as a common basic structure of practicalsemiconductor lasers.

[0006] Furthermore, in practical semiconductor lasers, various stripestructures are used in the horizontal direction parallel to the p-njunction surface to give the lasers the waveguide properties ofconfining a current or injected carriers into a stripe region in thatdirection, thereby stabilizing the transverse mode of oscillated laserlight.

[0007] This stripe active region is sometimes buried in a cladding layerregion having a larger forbidden band. The result is a buriedheterostructure (BH) in which the double heterostructure is also formedin th horizontal direction.

[0008] In this buried heterostructure, carriers are also confined in thehorizontal direction, resulting in an increased current injectionefficiency.

[0009] Furthermore, a two-dimensional optical waveguide is formed by therefractive index difference between the active layer and the claddingregion in which the active layer is buried. Consequently, it is possibleto obtain a fundamental transverse mode semiconductor laser with a highefficiency and a stable oscillation mode.

[0010] One example of the means of forming a buried layer to obtain aburied heterostructure is HVPE (Hydride (Chloride) Vapor Phase Epitaxy)which makes use of a difference from thermal equilibrium by using anitrogen- or hydrogen-diluted gas mixture of a Group V gas, such as PH₃or PCl₃, and a Group III gas of, e.g., a metal halide (InCl) formed by athermo-chemical reaction with HCl.

[0011] MOVPE (Metal Organic Vapor Phase Epitaxy) is also available inwhich a gasified (diluted) organic metal (primarily of Group III) iscrystal-grown together with a Group V gas (PH₃) by substrate heating.

[0012] Alternatively, the temperature of a solution containing asemiconductor material is decreased by bringing the solution intocontact with the substrate surface. Consequently, the liquid phase inthe boundary region supersaturates and precipitates (crystal-grows) onthe substrate. This method is called LPE (Liquid Phase Epitaxy).

[0013]FIGS. 29A and 29B are schematic cross-sectional views showing thearrangements of semiconductor lasers having an SIBH (Semi-InsulatingBuried Hetero) structure formed by selectively burying semi-insulatingInP using HVPE.

[0014] This semiconductor laser is fabricated as follows.

[0015] First, an n-type InP cladding layer 2 is crystal-grown on ann-type InP substrate 1 of (001) orientation by using MOCVD or MBE(Molecular Beam Epitaxy). Examples of the dopant for obtaining n-typeare Se, Si, and S.

[0016] Subsequently, an active layer 3 is formed on top of thestructure.

[0017] This active layer 3 consists of a guide layer (light confininglayer) made from, e.g., undoped or n-type-doped InGaAsP, an active layerformed on the guide layer and made from undoped InGaAsP, and a guidelayer formed on the active layer and made from undoped or p-type-dopedInGaAs.

[0018] Subsequently, a p-type InP overcladding layer 4 is formed on theactive layer 3. Examples of the dopant for obtaining a p-type layer areZn and Be.

[0019] A p-type InGaAs or InGaAsP electrode contacting layer 5 is thenformed on the overcladding layer 4. This electrode contacting layer 5 isformed to obtain an ohmic contact (and to decrease the contactresistance) with an electrode (to be described later).

[0020] An InP layer is sometimes formed on the electrode contactinglayer 5 to protect the electrode layer or increase the adhesion of amask material. However, no such layer is used in this structure.

[0021] Subsequently, a stripe pattern (not shown) made from siliconoxide is formed on the electrode contacting layer 5 by photolithographyand etching. This stripe pattern is used as a mask to perform etching toa portion below the active layer, forming a stripe etching mesa.

[0022] A semi-insulating InP buried layer 6 is then formed to bury theboth sides of the etching mesa by using Fe as a dopant. This formationis done by HVPE as described above.

[0023] A p-type electrode 7 consisting of an Au—Zn—Ni alloy is formed onthe buried layer 6, and an n-type electrode consisting of an Au—Ge—Nialloy is formed on the lower surface of the substrate 1. Consequently, asemiconductor laser with the structure shown in FIG. 29A is formed.

[0024] Note that as th p-type electrode 7, a Ti—Pt—Au alloy can also beSchottky-connected in some instances.

[0025]FIG. 29B is a sectional view showing the arrangement of asemiconductor laser formed using an overcladding layer 4 a and anelectrode contacting layer 5 a both increased in area to increase theinjection efficiency of carriers into an active layer 3.

[0026] In FIG. 29B, reference numeral 9 denotes an n-type InP currentblocking layer for suppressing a recombination current with capturedelectrons resulting from injection of holes from the overcladding layer4 a into a semi-insulating layer 6; and 10, an insulating layer madefrom silicon oxide or silicon nitride. The rest of the arrangement issimilar to that in FIG. 29A.

[0027] Note that the current blocking layer 9 is usually formed usingthe same growth apparatus (e.g., an MOCVD or MOVPE apparatus) as for thep-type overcladding layer 4 a.

[0028] On the other hand, as a semiconductor laser in which a buriedlayer is formed by LPE or MOCVD (MOVPE), semiconductor lasers having ap-n buried structure are available in which, as illustrated in FIGS. 30Aand 30B, conductive carriers are confined by forming a p-n junctionbarrier.

[0029] In the structure shown in FIG. 30A, the both sides of a claddinglayer 2 and an active layer 3, as an etching mesa, are buried with ap-type InP current blocking layer 11 and an n-type InP current blockinglayer 12 by using LPE or MOCVD. Thereafter, a p-type overcladding layer4 a is grown.

[0030]FIG. 30B is a sectional view showing the arrangement of asemiconductor laser with a DCPBH (Double Channel Planar Buried Hetero)structure.

[0031] In this structure, an etching mesa is not singly formed; that is,an etching mesa consisting of a buffer layer 2 and an active layer 3 isformed by forming trenches.

[0032] These trenches are buried with current blocking layers 11a and 12a.

[0033] Note that the same reference numerals as in FIGS. 29A and 29Bdenote the same portions in FIGS. 30A and 30B.

[0034] Also, as with the waveguide lasers described above, verticalresonator type surface emission lasers which vertically emit a laserlight from the substrate surface must also have the buried structure toimprove the performance.

[0035]FIG. 31 is a sectional view showing the major components of asurface emission laser with a GaAs buried structure.

[0036] As shown in FIG. 31, this surface emission laser has acylindrical active region 301 and two reflecting mirrors 302 parallel toeach other in the vertical direction. These reflecting mirrors 302 havea DBR (Distributed Bragg Reflector) structure.

[0037] The active region 301 is processed into a cylindrical mesastructure 303 by a chemical etching solution and buried with a buriedlayer 304 formed by LPE.

[0038] The buried layer 304 has a pnp structure, and so a current in thetransverse direction is blocked.

[0039] In FIG. 31, reference numerals 310 and 311 denote claddinglayers; 312, a diffused region; 313, an electrode; 314, a buffer layer;315, a GaAs substrate; and 316, an electrode.

[0040] In burying the cylindrical or square active region in thissurface emission laser with the above arrangement, the conventionalapproach is to etch the entire region except for the region to be buriedand bury that region using epitaxy such as LPE.

[0041] Recently, however, the burying technique using semi-insulatingInP has been developed and is beginning to be used as described earlier.

[0042] Burying with this semi-insulating InP can improve the performanceof a laser, e.g., can increase the modulation rate and decrease theoscillation threshold value.

[0043] The application of this technique is expected to improve theperformance of surface emission lasers and other surface type opticalelements, as well as waveguide lasers.

[0044] Unfortunately, the above conventional structures have thefollowing problems.

[0045] First, the formation of the SIBH structure using VPE describedabove is restricted by the structure of a waveguide (etching mesa) to beburied.

[0046] As an example, in the formation of a buried layer using MOVPE,the burying of a waveguide extending in the (011) direction is common.

[0047] If, however, this is used to bury a waveguide, such as a crossedwaveguide or a surface type optical element, having a structure with twoor more combinations of different orientations, the reaction sitediffers from one orientation to another. As a result, around a waveguidein the (0{overscore (1)}1) direction, an abnormal growth such as anoverhang readily occurs on the waveguide. This makes flat burying growthimpossible.

[0048] On the other hand, such an abnormal growth hardly occurs in HVPEsince HVPE is a thermal equilibrium process. However, the orientationdependence of the growth rate significantly appears near a regrowthtemperature of 650° C. That is, growth in the transverse directionbecomes dominant. Accordingly, a slow-growing (001) face is formed inthe regrowth process, and this forms a flat surface.

[0049]FIGS. 32A and 32B are perspective views showing the arrangement ofa crossed waveguide in which a waveguide whose side surfaces are in the(011) direction and a waveguide whose side surfaces are in the(0{overscore (1)}1) direction cross each other.

[0050] In FIGS. 32A and 32B, reference numeral 6 b denotes a buriedlayer being grown; and 13, a selective growth mask made from siliconoxide. The rest of the structure is analogous to that shown in FIGS. 29Aand 29B.

[0051] In this structure, the crystal growth rate of the buried layer 6b in the (011) direction is greatly different from that in the(0{overscore (1)}1) direction; the crystal growth rate around thewaveguide in the (0{overscore (1)}1) direction is very high.

[0052] The present inventors have also found that the growth rate aroundthe waveguide in the (011) direction, in which the growth rate isoriginally low, is further lowered in the crossed structure whencompared with that in the (011) direction due to mass-transport of areactant (INCl) and a by-product (HCl) in a surface reactionrate-determining process (kinetic control) under near-equilibriumsystem.

[0053] Consequently, when the both sides of this crossed waveguide areburied with a buried layer, a portion around the waveguide in the (011)direction is not much buried as illustrated in FIG. 32B.

[0054] In contrast, the waveguide in the (0{overscore (1)}1) directionis easily covered with the buried layer, making it difficult to performflat burying growth even with HVPE.

[0055] Vertical resonator type surface emission lasers also have theproblem of difficulty in flat burying growth, because the side walls ofthese lasers consist of a large number of orientations.

[0056]FIGS. 33A to 33C are views for explaining a method of buryingsquare mesa structures by using HVPE described previously. FIG. 33A is aplan view, FIG. 33B is a sectional view taken along the (011) direction,and FIG. 33C is a sectional view taken along the (0{overscore (1)}1)direction.

[0057] No flat surface can be obtained as described above since thegrowth in the horizontal direction from side walls 335 of a buried mesastructure 333 is faster than the growth in the vertical direction from abottom surface 336 and since the growth rate changes in accordance withthe orientation of the side walls 335.

[0058] On the other hand, the formation of a p-n buried structure usingLPE has the problems of, e.g., the production of a junction capacitanceand the production of a leakage current from the burying interface, andthe limitations on the depth of burying for obtaining a pnp layeredstructure

[0059] Additionally, LPE has a high processing temperature of 700° C. ormore, and this degrades the performance of buried elements.

SUMMARY OF THE INVENTION

[0060] It is, therefore, a principal object of the present invention toprovide an optical semiconductor device having a buried heterostructurewith a flat surface and a method of fabricating the same.

[0061] To achieve the above object according to an aspect of the presentinvention, there is provided an optical semiconductor device comprisingan optical semiconductor element formed on a semiconductor substrate, asemiconductor region opposing the optical semiconductor element andessentially surrounding the optical semiconductor element to form walls,and a buried layer arranged between the walls of the semiconductorregion and the optical semiconductor element and formed by vapor phaseepitaxy, wherein a distance between the wall of the semiconductor regionand a side wall of the optical semiconductor element is larger in aportion in which a growth rate of the vapor phase epitaxy in ahorizontal direction from the side wall of the optical semiconductorelement and the wall of the semiconductor region is higher.

[0062] According to another aspect of the present invention, there isprovided an optical semiconductor device comprising a plurality ofoptical semiconductor elements formed on a semiconductor substrate, adummy portion formed in a center of a square at four corners of whichthe optical semiconductor elements are arranged, and a buried layerformed by vapor phase epitaxy so as to bury a portion between theoptical semiconductor elements and the dummy portion.

[0063] According to still another aspect of the present invention, thereis provided a method of fabricating an optical semiconductor device,comprising the first step of forming an optical semiconductor element ona semiconductor substrate, the second step of forming a semiconductorregion having walls opposing the optical semiconductor element andessentially surrounding the optical semiconductor element, and the thirdstep of forming a buried layer by vapor phase epitaxy between the wallsof the semiconductor region and the optical semiconductor element,wherein in the second step, a distance between the wall of thesemiconductor region and a side wall of the optical semiconductorelement is larger in a portion in which a growth rate of the vapor phaseepitaxy in a horizontal direction from the side wall of the opticalsemiconductor element and the wall of the semiconductor region ishigher.

[0064] According to still another aspect of the present invention, thereis provided a method of fabricating an optical semiconductor device,comprising the steps of forming a plurality of optical semiconductorelements on a semiconductor substrate, forming a dummy portion in acenter of a square at four corners of which the optical semiconductorelements are arranged, and forming a buried layer by vapor phase epitaxyso as to bury a portion between the optical semiconductor elements andthe dummy portion.

[0065] The optical semiconductor device of the present inventioncomprises an optical semiconductor element formed on a semiconductorsubstrate, a semiconductor region opposing the optical semiconductorelement and essentially surrounding the optical semiconductor element toform walls, and a buried layer arranged between the walls of thesemiconductor region and the optical semiconductor element and formed byvapor phase epitaxy.

[0066] A distance between the wall of the semiconductor region and aside wall of the optical semiconductor element is determined inaccordance with the growth rate of vapor phase epitaxy which isdetermined by the directions, which are perpendicular to the orientationof the semiconductor substrate, of the side wall of the opticalsemiconductor element and the wall of the semiconductor region, and bythe orientation of the semiconductor substrate.

[0067] Accordingly, in the optical semiconductor device consisting ofthe optical semiconductor element having side surfaces different indirection, the distance between the side wall of the opticalsemiconductor element and the wall of the semiconductor region isdetermined in relation to the growth rate of the buried layer. As aresult, the buried layer is formed flat.

[0068] Also, the optical semiconductor device of the present inventioncomprises a plurality of optical semiconductor elements formed on asemiconductor substrate, a dummy portion formed in the center of asquare at the four corners of which the optical semiconductor elementsare arranged, and a buried layer formed by vapor phase epitaxy so as tobury a portion between the optical semiconductor elements and the dummyportion.

[0069] Accordingly, the buried layer is formed by growth from the sidesurfaces of the dummy portion in addition to growth from the surface ofthe semiconductor substrate and from the side surfaces of the opticalsemiconductor elements.

[0070] The method of fabricating an optical semiconductor deviceaccording to the present invention comprises the first step of formingan optical semiconductor element on a semiconductor substrate, thesecond step of forming a semiconductor region having walls opposing theoptical semiconductor element and essentially surrounding the opticalsemiconductor element, and the third step of forming a buried layer byvapor phase epitaxy between the walls of the semiconductor region andthe optical semiconductor elements.

[0071] In the second step, a distance between the wall of thesemiconductor region and a side wall of the optical semiconductorelement is determined in accordance with the growth rate of vapor phaseepitaxy which is determined by the directions, which are perpendicularto the orientation of the semiconductor substrate, of the side wall ofthe optical semiconductor element and the wall of the semiconductorregion, and by the orientation of the semiconductor substrate.

[0072] Accordingly, in the optical semiconductor device consisting ofthe optical semiconductor element having side surfaces different indirection, the distance between the side wall of the opticalsemiconductor element and the wall of the semiconductor region isdetermined in relation to the growth rate of the buried layer. As aresult, the buried layer is formed flat.

BRIEF DESCRIPTION OF THE DRAWINGS

[0073]FIGS. 1A to 1C are a plan view and sectional views for explaininga method of fabricating an optical semiconductor device as the firstembodiment of the pr sent invention;

[0074]FIGS. 2A and 2B are sectional views for explaining the method offabricating the optical semiconductor device as the first embodiment ofthe present invention;

[0075]FIGS. 3A and 3B are scanning electron micrographs showing thesurface conditions of a buried structure for explaining the method offabricating the optical semiconductor device as the first embodiment ofthe present invention;

[0076]FIGS. 4A and 4B are schematic cross-sectional views in the (011)and (0{overscore (1)}1) direction, respectively, when a crossedwaveguide structure, in which the distance between the wall of asemiconductor region and a semiconductor element in the (011) directionequals that in the (0{overscore (1)}1) direction, is grown by burying asemiconductor using HVPE;

[0077]FIGS. 5A and 5B are sectional views for explaining the method offabricating the optical semiconductor device as the first embodiment ofthe present invention;

[0078]FIGS. 6A to 6C are sectional views showing the arrangement of anoptical semiconductor device according to the second embodiment of thepresent invention;

[0079]FIGS. 7A and 7B are a plan view and a sectional view,respectively, showing the arrangement of an optical semiconductor deviceaccording to the third embodiment of the present invention;

[0080]FIGS. 8A to 8D are sectional views showing a method of fabricatingan optical semiconductor device as the fourth embodiment of the presentinvention;

[0081]FIGS. 9A to 9D are a perspective view and plan views showing thearrangement of an optical semiconductor device according to the fifthembodiment of the present invention;

[0082]FIGS. 10A to 10E are a perspective view, plan views, and asectional view showing the arrangement of an optical semiconductordevice according to the sixth embodiment of the present invention andthe arrangement of the device during the fabrication;

[0083]FIG. 11 is a view showing a method of burying a mesa structure forexplaining one embodiment of a semiconductor device fabrication methodaccording to the present invention;

[0084]FIG. 12A is a sectional view of the buried mesa structure (FIG.11) viewed in the (0{overscore (1)}1) direction;

[0085]FIG. 12B is a sectional view of the mesa structure viewed in the(011) direction;

[0086]FIGS. 13A and 13B are sectional views showing a method of buryinga mesa structure for explaining another embodiment of the semiconductordevice fabrication method;

[0087]FIG. 14 is a plan view showing the state in which a plurality ofthe mesa structures in FIGS. 13A and 13B are arranged;

[0088]FIG. 15 is a plan view for explaining another method of burying amesa structure in the semiconductor device fabrication method of thepresent invention;

[0089]FIG. 16 is a plan view showing a method of burying a mesastructure for explaining still another embodiment of the semiconductordevice fabrication method of the present invention;

[0090]FIG. 17 is a plan view showing a method of burying a mesastructure for explaining still another embodiment of the semiconductordevice fabrication method of the present invention;

[0091]FIGS. 18A to 18D are plan and sectional views showing thearrangement of an optical semiconductor device during the fabrication,in which an array of a plurality of optical elements is formed;

[0092]FIGS. 19A to 19F are plan and sectional views showing thearrangement of an optical semiconductor device according to the 10thembodiment of the present invention, in which an array of a plurality ofoptical elements is formed;

[0093]FIGS. 20A to 20C are plan views showing the arrangement of anoptical semiconductor device according to the 11th embodiment of thepresent invention, in which an array of a plurality of optical elementsis formed;

[0094]FIG. 21 is a perspective view showing part of a process offabricating a semiconductor laser having electrode and wiring regionsfor explaining the arrangement of an optical semiconductor deviceaccording to still another embodiment of the present invention;

[0095]FIG. 22 is a perspective view of a semiconductor laser forexplaining the arrangement of a semiconductor device according to stillanother embodiment of the present invention;

[0096]FIG. 23 is a perspective view showing part of a process offabricating a semiconductor laser having electrode and wiring regionsfor explaining the arrangement of a semiconductor device according tostill another embodiment of the present invention;

[0097]FIG. 24 is a perspective view showing the arrangement of atransverse light injection type bistable laser as a semiconductor deviceaccording to still another embodiment of the present invention;

[0098]FIG. 25 is a perspective view showing the arrangement of asemiconductor mode-locked laser as a semiconductor device according tostill another embodiment of the present invention;

[0099]FIG. 26 is a perspective view showing the arrangement of awaveguide photodetector as a semiconductor device according to stillanother embodiment of the present invention;

[0100]FIG. 27 is an enlarged sectional view for explaining a remaininglaser region, an electrode metal, and an insulating structure accordingto the semiconductor device in FIG. 21;

[0101]FIG. 28 is an enlarged sectional view for explaining the remaininglaser region, the electrode metal, and the insulating structureaccording to the semiconductor device in FIG. 21;

[0102]FIGS. 29A and 29B are sectional views showing the arrangements ofconventional semiconductor lasers;

[0103]FIGS. 30A and 30B are sectional views showing the arrangements ofconventional semiconductor lasers;

[0104]FIG. 31 is a sectional view showing the arrangement of aconventional semiconductor laser;

[0105]FIGS. 32A and 32B are perspective views for explaining aconventional semiconductor laser fabrication method;

[0106]FIGS. 33A to 33C are views for explaining a conventionalsemiconductor laser fabrication method;

[0107]FIG. 34A is a scanning electron micrograph showing the surfacecondition of a formed portion for explaining the growth state of aburied layer in the present invention; and

[0108]FIG. 34B is a scanning electron micrograph showing the surfacecondition of the formed portion for explaining the growth state of theburied layer in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0109] The first embodiment of the present invention will be describedin detail below with reference to the accompanying drawings.

[0110] First Embodiment

[0111]FIGS. 1A to 1C, 2A and 2B, and 5A and 5B are a plan view andsectional views for explaining a method of fabricating an opticalsemiconductor device as the first embodiment of the present invention.

[0112] In FIGS. 1A, 1B, and 1C, reference numeral 13 denotes a siliconoxide selective growth mask formed by photolithography and etching; 14,a trench whose side walls are in the (011) direction; 15, a trench whoseside walls are in the (0{overscore (1)}1) direction; and 16, a dummyportion (a semiconductor region for forming walls). The rest of thearrangement is analogous to that shown in FIG. 32. That is, the trench14 extends in the (0{overscore (1)}1) direction, and the trench 15extends in the (011) direction.

[0113]FIG. 1B is a sectional view taken along the line A-A′ in the(0{overscore (1)}1) direction, and FIG. 1C is a sectional view takenalong the line B-B′ in the (011) direction.

[0114] Note that the orientation of a substrate 1 is (001).

[0115] Although not shown, the selective growth mask 13 may be form daft r an InP layer is formed on an electrode contacting layer 5.

[0116] With this arrangement, it is possible to protect the electrodecontacting layer 5 and improve the adhesion of the selective growth mask13.

[0117] In FIGS. 1A to 1C, stripe portions containing an active layer 3between the trenches 14 and 15 serve as waveguides. Side walls opposingthe side walls of these stripe portions are the walls of this stripeoptical semiconductor element.

[0118] Note that the plan view and the sectional views are not at thesame scale.

[0119] In this embodiment, layers up to the electrode contacting layer 5are first formed as in the conventional semiconductor lasers illustratedin FIGS. 29A and 29B.

[0120] Subsequently, the selective growth mask 13 made from siliconoxide is formed on the electrode contacting layer 5 and used as a maskto selectively etch the electrode contacting layer 5, an overcladdinglayer 4, the active layer 3, a cladding layer 2, and the substrate 1.

[0121] This etching is done by reactive ion etching using a plasma whichis formed by performing RF glow discharge for a hydrocarbon gas such asmethane gas or thane gas, or a halogen gas such as bromine gas orchlorine gas.

[0122] Note that in this first embodiment, the etching is so performedas to leave the dummy portion 16, and the trench 14 is made wider thanthe trench 15.

[0123] The carbon decomposition product adhered to the side and bottomsurfaces of the trenches 14 and 15 by this etching treatment is removedby a dry process using oxygen plasma and a wet process using sulfuricacid, thereby cleaning the etched surfaces.

[0124] As a result, as shown in FIGS. 1A, 1B and 1C, the trench 14having the side surfaces perpendicular to the (011) direction is madewider, and the trench 15 having the side surfaces whose directionperpendicular to the orientation of the substrate 1 is the (0{overscore(1)}1) direction is made narrower. In this state, the waveguidessandwiched between the trenches 14 and 15 are formed to cross each otherin the (0{overscore (1)}1) and (011) directions.

[0125] Subsequently, as illustrated in FIGS. 2A and 2B, InP 21 iscrystal-grown in the trenches 14 and 15 by using HVPE.

[0126] In this crystal growth, a Group V element gas such as PH₃ or PCl₃and a Group III element gas such as InCl are used as source gases, and achloride (ferrous chloride: FeCl₂) of Fe is mixed to dope Fe. InCl andFeCl₂ are produced by placing In and Fe in a high-temperature region andsupplying HCl to that region. HCl is diluted with nitrogen and used as acarrier.

[0127] The partial pressure of the Group V element gas is controlled tobe higher than the partial pressure of the Group III element gas, andthereby the growth mode is rate-determined by InCl as a Group IIIcompound even in the system in which different orientations, such as(111)A and (111)B surfaces, are mixed. Consequently, the crystal growthin the horizontal direction from the side surfaces of the trenches 14and 15 is made dominant. Additionally, the efficiency at which Fe, whichis activated in the reaction site of the Group III element, isincorporated is increased, and the growth in the (001) orientation ofthe substrate 1 is relatively suppressed.

[0128] The doped Fe forms the ground state of deep acceptors at aposition of about −0.6 eV from the bottom of the conduction band in thebad gap of InP.

[0129] Additionally, InP is made semi-insulating by activating Fe at1×10¹⁷ cm⁻³ at which injected electrons are well captured, even now thebackground carrier concentration in an undoped state, resulting from ann-type impurity incorporated during the growth, to an order of 1×10¹⁵cm⁻³ or lower, thereby compensating it.

[0130] As a consequence, the Fe-doped InP 21 is crystal-grown from theside surfaces of the trench s 14 and 15, and there by the trenches 14and 15 are buried.

[0131] As shown in FIGS. 2A and 2B, on the side surfaces of the trench14 of which the side walls 1 are perpendicular to the (011) direction, aso-called A-surface system (primarily a (113) A surface) in which thesurface is terminated by In during regrowth appears. Therefore, on theside surfaces of the trench 14 the growth of the InP 21 is made fasterby the surface reaction rate-determination than on the side surfaces ofthe trench 15 of which the side walls are perpendicular to the(0{overscore (1)}1) direction, i.e., in the direction in which aso-called B-surface system (primarily a (113) B surface) in which thesurface is terminated by P during regrowth appears.

[0132] Also, when growth surfaces having different growth rates areclose to each other such as in a crossed waveguide, HCl produced by athermal equilibrium reaction:

PH₃+InCl→InP(A surface)+H₂+HCl

[0133] on the growth surface whose growth rate is increased by thekinetic controlled process also diffuses into the slow-growing surfaceby mass-transport. This suppresses a reaction:

PH₃+InCl ⇄InP(B surface)+H₂+HCl

[0134] on this slow-growing surface.

[0135] Consequently, the trench in the (0{overscore (1)}1) direction isburied much faster than the tr nch in th (011) direction (FIG. 3A).

[0136] Thereafter, however, the (001) surface appears in the buriedtrench in the (0{overscore (1)}1) direction. Accordingly, the surfacereaction rate-determination process becomes dominant in the B-surfacesystem (particularly the (113) B surface) which appears in the trench inthe (011) direction, and supply of HCl from the trench in the(0{overscore (1)}1) direction is also decreased. Therefore, asillustrated in FIG. 3B, the trench in the (011) direction is immediatelyburied, and this prevents a rise of InP growing in the trench in the(0{overscore (1)}1) direction and serving as a buried layer.

[0137] This is so because the trench in the (0{overscore (1)}1)direction corresponding to the trench 14 in FIGS. 2A and 2B is madewider, and this makes it possible to take matching between thekinetic-process described previously and the accompanyingmass-transport.

[0138] In contrast, if the widths of the two trenches are made equal oralmost equal to each other, as shown in FIGS. 4A and 4B, the trench(FIG. 4B) in the (0{overscore (1)}1) direction is buried very fast, andgrowth occurs even while the trench in the (011) direction whichrequires a relatively long time is being buried. The result is thatgrowing InP 41 overhangs to impair the flatness.

[0139] As described above, according to this embodiment, both thetrenches 14 and 15 (FIGS. 2A and 2B) are buried flat.

[0140] Thereafter, the insulating film 13 is removed, and an InP layer(not shown), if it is formed on the electrode contacting layer 5, isselectively removed by wet etching. In this wet etching, since only InPis etched away, the buried layer 6 also is slightly etched.

[0141] Subsequently, as illustrated in FIGS. 5A and 5B, an insulatingfilm 10 is so formed as to expose the waveguide region containing theburied layer 6. Finally, a p-type electrode 7 is formed by selectivelyvapor-depositing a metal on the insulating film 10.

[0142] Second Embodiment

[0143] In the above first embodiment, the trenches in which the buriedlayer is formed are formed by dry etching (reactive ion etching).However, the present invention is not limited to this embodiment.

[0144] That is, these trenches can also be similarly formed by wetetching having no directivity and/or no selectivity (to a semiconductorlayer: e.g., InP, InGaAs, or InGaAsP).

[0145]FIGS. 6A to 6C are sectional views showing the arrangement of anoptical semiconductor device according to the second embodiment of thepresent invention, in which trenches 14 a and 15 a are formed by wetetching.

[0146] In FIGS. 6A to 6B, reference numerals 14 a and 15 a denotetrenches formed by etching using methanol bromide, and so did another inFIG. 6C using aqueous potassium dichromate solution. The rest of thearrangement is identical with that shown in FIGS. 5A and 5B.

[0147] This etching reflects the relationship between the (011)direction and the (100) direction and the relationship between the(0{overscore (1)}1) direction and the (100) direction.

[0148] As a result, in the formation of the trench 15 a in which thestripe direction is (011), in other words, in which the direction of theside surfaces is the (0{overscore (1)}1) direction in the case of dryetching, a waveguide becomes an inverted as illustrated in FIG. 6A.

[0149] On the side walls, a (111) A surface appears from a mask in theupper portion to the substrate, and then a B-surface system in which the(111) surface gradually changes to the (001) surface appears. Duringregrowth, growth from the (111) A surface first occurs and in the earlystages, the inverted changes to a non-inverted whose surface is theB-surface system.

[0150] On the other hand, in the formation of the trench 14 a in whichthe stripe direction is (0{overscore (1)}1), in other words, in whichthe direction of the side surfaces is the (011) direction in the dryetching case, a waveguide becomes a non-inverted as illustrated in FIG.6B. Since the side walls are terminated by In as previously described inthe first embodiment, the growth rate on thes walls is very high(kinetic control).

[0151] As described above, even when a non-inverted and an inverted areformed on the same substrate in accordance with the directions offormation of stripe waveguides, the trench in which the side surfacesare in the (011) direction is made wider than the trench in which theside surfaces are in the (0{overscore (1)}1) direction. As aconsequence, a buried layer 6 is formed flat with no overhang in eithertrench.

[0152] Also, as shown in the perspective view of FIG. 6C, when isotropicdry etching is used in the formation of the trenches, the trench on theside surfaces of which crystal growth occurs faster is made wider forthe same reasons as above. As a consequence, the buried layer forburying waveguides is formed flat in either trench in the same manner asdescribed above.

[0153] Third Embodiment

[0154] In the above embodiments, crossed waveguides are formed on thesame substrate. However, the present invention is not restricted tothese embodiments.

[0155] That is, these waveguides can also be similarly formed in amatrix form.

[0156]FIGS. 7A and 7B are a plan view and a sectional view,respectively, showing the arrangement of an optical semiconductor devicein which waveguides are formed in a matrix form.

[0157]FIG. 7B shows the section taken along the line A-A′ in FIG. 7A inwhich a p-type electrode 7 and an insulating film 10 shown in FIG. 7Bare not formed. Also, FIGS. 7A and 7B are not at the same scale, andFIG. 7B illustrates a waveguide 71 at the center.

[0158] In FIGS. 7A and 7B, reference numeral 14 b denotes a trench whoseside surfaces are in the (011) direction; 15 b, a trench whose sidesurfaces are in the (0{overscore (1)}1) direction; 71, waveguides; and72, a dummy portion. The rest of the arrangement is identical with thatshown in FIGS. 5A and 5B.

[0159] Even in burying the waveguides 71 as in this embodiment, etchingis so performed as to leave the dummy portion 72 to thereby form thetrenches 14 b and 15 b in the same way as in the previous embodiments.

[0160] As in the above embodiments, the trench 14 b is formed to have awidth of about 100 μm, i.e., to be wider than the trench 15 b. That is,as illustrated in the plan view of FIG. 7A, the dummy portion 72 isformed to be as thin as the waveguides 71.

[0161] After the trenches 14 b and 15 b are formed as described above, aburied layer 6 is formed in the same fashion as in the first embodiment.The insulating film 10 is then so formed as to cover the upper portionof the dummy portion 72, and the p-type electrode 7 is formed in amatrix form so as to be connected to an electrode contacting layer 5.

[0162] Fourth Embodiment

[0163] In the above third embodiment, in burying the matrix waveguides,the dummy portion analogous to the waveguides is formed in each squareof the matrix. However, the present invention is not limited to this.

[0164]FIGS. 8A to 8D are sectional views showing a method of fabricatingan optical semiconductor device according to the fourth embodiment ofthe present invention.

[0165]FIGS. 8A to 8C show the section taken along the line A-A′ of FIG.8D which illustrates the state corresponding to FIG. 8B. Note that thesectional views and the plan view are not at the same scale.

[0166] The fabrication method of this fourth embodiment will bedescribed below.

[0167] First, as in the previous embodiments, layers up to an electrodecontacting layer 5 are formed as illustrated in FIG. 8A. Subsequently,an insulating film 13 is so formed that waveguides 81 to be formed in amatrix form and a dummy portion 82 to be formed in each square of thematrix are left behind. The insulating film 13 is used as a mask toselectively etch the electrode contacting layer 5 and an overcladdinglayer 4.

[0168] The insulating film 13 on the dummy portions 82 is thenselectively removed.

[0169] Subsequently, with the insulating film 13 serving as a mask notpresent on the dummy portions 82, etching is performed again to form thewaveguides 81 such that, as illustrated in FIG. 8B, the dummy portions82 etched away to an active layer 3 are formed in the individual squaresof the matrix.

[0170] In this structure, a trench 14 c is made wider than a trench 15c.

[0171] In this state, InP is deposited to form a buried layer 6 a underthe same conditions as in the first embodiment, and a p-type electrode 7is formed on the buried layer 6 a so as to be connected to the electrodecontacting layer 5 of the waveguides 81.

[0172] According to this embodiment as described above, the buried layer6 with a gentle slope is formed from the periphery of the waveguide 81to the formation site of the dummy portion 82, so the dummy portion 82is buried in the buried layer 6 having semi-insulating properties.Therefore, the dummy portion 82 is not connected to the p-type electrode7, and this obviates the need for the insulating film 10 shown in FIGS.5A and 5B, 6A to 6C, and 7A and 7B.

[0173] Note that in FIGS. 8A to 8D, the same reference numerals as inFIGS. 7A and 7B denote the same parts.

[0174] Fifth Embodiment

[0175] The fifth embodiment of the present invention will be describedbelow.

[0176]FIGS. 9A to 9D are views showing a schematic arrangement of adirectional coupler in which two waveguides 91 and 92 are partiallyclose to each other.

[0177] In FIGS. 9A to 9D, reference numerals 93 and 94 denote dummyportions, and the same numerals as in FIGS. 1A to 1C denote the sameparts.

[0178] The spacing between the close parallel portions of the twowaveguides 91 and 92 is as small as 2 μm or smaller to permitpropagation of light. However, other regions must confine light.

[0179] For this purpose, in the formation of this directional coupler,an etching mask having a slit 95 shown in FIG. 9B is first used to forma trench between the parallel portions. After this trench is buried,trenches in other portions are formed. Note that FIGS. 9A, 9C, and 9Dshow a state in which the trenches are formed between the parallelportions.

[0180] In this structure, the direction of the side surfaces of theadjacent parallel portions of the waveguides 91 and 92 is different fromthat in other portions.

[0181] If the direction of the parallel portions in which the waveguides91 and 92 are close to each other is (011), a B-surface system appearson the side surfaces of the parallel portions during growth as describedin the first embodiment, resulting in a low growth rate.

[0182] If this is the case, therefore, as illustrated in the plan viewof FIG. 9C, the dummy portions 93 and 94 are so formed that the trenchwidth is slightly decreased in the parallel portions.

[0183] Note that the methods of forming and burying the trenches forburying growth are analogous to those in the first embodiment.

[0184] On the other hand, if the direction of the parallel portions inwhich the waveguides 91 and 92 are close to each other is (0{overscore(1)}1), an A-surface system. appears on the side surfaces of theparallel portions during growth as described in the first embodiment.The result is that the crystal grows faster.

[0185] If this is the case, therefore, as shown in the plan view of FIG.9D, the dummy portions 93 and 94 are so formed that the trench width isincreased in the parallel portions.

[0186] Sixth Embodiment

[0187] The sixth embodiment of the present invention will be describedbelow.

[0188]FIGS. 10A to 10E are a perspective view, plan views, and asectional view showing the arrangement of an optical semiconductordevice having a branching waveguide such as a Mach-Zehnder waveguideaccording to this sixth embodiment.

[0189] In FIGS. 10A to 10E, reference numeral 101 denotes a waveguidewhich once branches into branching waveguides 101 a and 101 b whichagain merge into a single waveguide.

[0190] Reference numeral 102 denotes a dummy portion for the branchingwaveguide 101 a; and 103, a dummy portion for the branching waveguide101 b. The rest of the arrangement is similar to that shown in FIGS. 9Ato 9D.

[0191] If the direction of the waveguide 101 is (0{overscore (1)}1), anA-surface system appears on the side surfaces of this portion duringgrowth, with the result that the crystal grows fast on these sidesurfaces.

[0192] If this is the case, therefore, as shown in the plan view of FIG.10B, the trench width is increased in the parallel portions.

[0193] In portions of the branching waveguides 101 a and 101 b in whichthe direction is not the (0{overscore (1)}1) direction, the direction ofthe side surfaces is not the (011) direction. Therefore, in theseportions the spacing between the dummy portion 102 and the branchingwaveguide 101 a and the spacing between the dummy portion 103 and thebranching waveguide 101 b are narrowed.

[0194] On the other hand, if the direction of the waveguide 101 is(011), the crystal grows slowly on th sides surfaces of this portionsince a B-surface system appears on these side surfaces during growth.

[0195] If this is the case, therefore, as illustrated in the plan viewof FIG. 10C, the trench width is decreased in the parallel portions.

[0196] In portions of the branching waveguides 101 a and 101 b in whichthe direction is not the (011) direction, the side surfaces are not inthe (0{overscore (1)}1) direction. Accordingly, in these portions thespacing between the dummy portion 102 and the branching waveguide 101 aand the spacing between the dummy portion 103 and the branchingwaveguide 101 b are widened.

[0197] As shown in FIG. 1D which is the sectional view taken along theline A-A′of FIG. 10C, a spacing d₁ between the branching waveguides 101a and 101 b is wider than a spacing d₂ between the dummy portion 102 andthe branching waveguide 101 a and between the dummy portion 103 and thebranching waveguide 10 b.

[0198] As illustrated in the plan view of FIG. 10E, however, in whichFIG. 10E indicates the state of a buried layer 6 during the crystalgrowth, the portion between the branching waveguides 101 a and 101 b isburied faster due to the growth from an oblique portion spreading fromthe coupled portion of the branching waveguides 101 a and 101 b.

[0199] For this reason, the spacing d₁ is made wider than the spacingd₂.

[0200] Note that the methods of forming and burying the trenches forburying growth are analogous to those in the first embodiment.

[0201] Seventh Embodiment

[0202]FIGS. 11, 12A, and 12B are views for explaining a method ofburying a mesa structure in a vertical resonator type surface emissionlaser, according to the seventh embodiment of the semiconductor devicefabrication method of the present invention.

[0203]FIG. 11 is a plan view of the major components viewed from theabove, FIG. 12A is a sectional view taken along the (011) direction, andFIG. 12B is a sectional view taken along the (0{overscore (1)}1)direction.

[0204] As illustrated in FIGS. 33A to 33C described earlier, in buryingan array of a plurality of square mesa structures by using theconventional methods, the growth rate of a buried layer changes inaccordance with whether the side walls point in the (011) or(0{overscore (1)}1) direction. The result is that no flat surface can beattained.

[0205] In this embodiment, therefore, an optical semiconductor devicefabrication method will be described in which walls are formed around asquare mesa structure to make it possible to uniformly bury a buriedlayer and form a flat surface.

[0206] This is equivalent to forming the dummy portion 16 (semiconductorregion) in FIGS. 1A to 1C so as to surround a square mesa structure.

[0207] Note that in this seventh embodiment, it is assumed that asemiconductor used in burying is semi-insulating InP independently ofthe layered structure of a mesa structure to be buried. Note also that asemiconductor-stacked structure formed on an InP substrate and includingan active layer will be simply referred to as a substrate hereinafter.

[0208] First, as illustrated in the plan view of FIG. 11, an SiO₂ mask111 having a hole of a predetermined size is formed on the surface of asubstrate S having a (001) surface.

[0209] Assume the width of the hole of the mask 111 in the (011)direction is larger than the width in the (0{overscore (1)}1) direction.That is, a width L1 in the (011) direction is about 20 μm, and a widthL2 in the (0{overscore (1)}1) direction is about 8 μm.

[0210] This mask 111 is used as a mask to perform reactive ion etching,described in the first embodiment, for the substrate S, thereby forminga trench about 4 μm in depth D.

[0211] Consequently, a square mesa structure 112 and walls 113surrounding the mesa are formed (FIGS. 12A and 12B). These wallscorrespond to the dummy portions in FIGS. 1A to 1C.

[0212] In this structure, side walls 114 of the mesa structure 112 andside walls 115 of the walls 113 oppose each other. Note that FIG. 12Ashows the s ction viewed in the (0{overscore (1)}1) direction, and FIG.12B shows the section viewed in the (011) direction.

[0213] Subsequently, the mask 111 and the etched surfaces are cleaned byan oxygen plasma ashing process and a sulfuric acid treatment, andFe-doped InP is grown by HVPE. During the growth, the mask 111 serves asa selective growth mask and the growth time is approximately 16 minutes.

[0214] As a result, the trench is buried flat with an InP layer 116 inboth the sectional view in the (0{overscore (1)}1) direction of FIG. 13Aand the sectional view in the (011) direction of FIG. 13B.

[0215]FIG. 14 is a plan view showing the state in which a plurality ofthe mesa structures 112 buried flat with an InP layer 116 are formed.

[0216] Compared to the conventional structure in FIGS. 33A to 33B, themesa structures 112 are surrounded by the walls 113, and the region ofthe InP layer 116 is small.

[0217] The growth of InP from the side walls 114 and 115 in the (011)direction differs from that in the (0{overscore (1)}1) direction.Therefore, the width in the (0{overscore (1)}1) direction in which theside walls point is decreased in advance. Consequently, the InP layer116 is evenly formed, and the mesa structures 112 are buried flat.

[0218] Eighth Embodiment

[0219] In the above seventh embodiment, the shape of the mesa structure112 to be buried is a square.

[0220] When a mesa structure 151 is circular as shown in FIG. 15, it isonly necessary to form the planar shape of the inner surface of a wall152 into an ellipse. In this case, the major axis direction of thiselliptical shape is set in the (011) direction in which the growth rateis high. This is because the growth surface from the wall perpendicularto this direction forms an A-surface system, as described in the firstembodiment.

[0221] For example, as illustrated in FIG. 15, a spacing L₃ in the majoraxis direction is set to approximately 20 μm, and a spacing L₄ in theminor axis direction is set to approximately 8 μm. Note in this case,the depth of the trench, i.e., the height of the mesa structure 151 isabout 4μm.

[0222] Ninth Embodiment

[0223] In the above seventh and eighth embodiments, walls surrounding amesa structure (active region) are formed singly. However, the presentinvention is not limited to these embodiments.

[0224] For example, as illustrated in the plan view of FIG. 16 or theperspective view of FIG. 17, it is also possible to form triple walls168 ₁, 168 ₂, and 168 ₃ or 178 ₁, 178 ₂, and 178 ₃.

[0225] In FIG. 16, reference numerals 166 ₁, 166 ₂, and 166 ₃ denote thebottom surfaces of trenches formed between a mesa structure and the wall168 ₁, between the walls 168 ₁, and 168 ₂, and between the walls 168 ₂and 168 ₃. Similarly, in FIG. 17, reference numerals 176 ₁, 176 ₂, and176 ₃ denote the bottom surfaces of trenches formed between a mesastructure 177 and the wall 178 ₁, between the walls 178 ₁ and 178 ₂, andbetween the walls 178 ₂ and 178 ₃.

[0226] Four embodiments of burying of a mesa structure in a verticalresonator type surface emission laser have been described above.However, the layered structure of a mesa to be buried can take variousstructures, such as a modulator structure, other than the laserstructure.

[0227] Also, the growth rate in the horizontal direction from a mesastructure or walls is higher than the growth rate in the directionperpendicular to the substrate surface, since a surface (e.g., a (113) Asurface or a (111) B surface) having a growth rate higher than thegrowth rate of the (001) substrate surface appears. Accordingly,limitations on the spacings between the mesa structure and the walls areloose, so it is possible to realize various dimensions other than thosedescribed in the abov embodiments.

[0228] Tenth Embodiment

[0229]FIGS. 18A to 18D and 19A to 19F are sectional and plan viewsshowing the states of optical semiconductor devices during fabricationin each of which a plurality of optical elements (active regions)containing an active layer are formed into an array, like in the seventhto ninth embodiments described above.

[0230] In FIGS. 18A to 18D, reference numeral 181 denotes opticalelements formed into an array at intervals of about 100μm; and 182 and182 a, buried layers during crystal growth. The optical elements 181correspond to the waveguides in the first embodiment.

[0231]FIGS. 18B and 18D are sectional views taken along the (0{overscore(1)}1) direction and the (011) direction, respectively. Note that inFIGS. 18A to 18D, the same reference numerals as in FIGS. 1A to 1Cdenote the same parts.

[0232] As illustrated in FIGS. 18A and 18B, in this embodiment, as inthe previous embodiments, the growth of the buried layer 182 duringcrystal growth on the side surfaces perpendicular to the (011) directionof the optical elements 181 is faster than the growth on the sidesurfaces perpendicular to the (0{overscore (1)}1) direction.

[0233] Consequently, as shown in FIGS. 18C and 18D, portions between theside surfaces perpendicular to the (011) direction of the opticalelements 181 ar buried before portions between the side surfacesperpendicular to the (0{overscore (1)}1) direction of the opticalelements 181 are completely buried. The result is that no flat surfacecan be obtained.

[0234] In contrast, as illustrated in FIGS. 19A to 19F showing the stateof the this embodiment, a dummy portion 183 is formed at the center of asquare at the four corners of which four optical elements 181 arearranged into an array. With this arrangement, a buried layer 182 bhaving a flat surface can be obtained as shown in FIG. 19C.

[0235]FIGS. 19A to 19F illustrate a buried layer 182 b, when the dummyportion 183 is formed, from the state during formation to the state atthe completion of burying.

[0236]FIG. 19D is a sectional view taken along the line A-A′ in FIG.19A, FIG. 19E is a sectional view taken along the line B-B′ in FIG. 19B,and FIG. 19F is a sectional view taken along the line C-C′ in FIG. 19C.

[0237] As described in the previous embodiments, the dummy portion 183can be formed simultaneously with the optical elements 181.

[0238] When the dummy portion 183 is thus formed, the distance betweenthe side surfaces perpendicular to the (0{overscore (1)}1) direction ofthe optical elements 181 becomes short r than the distance between theside surfaces perpendicular to the (011) direction.

[0239] As a consequence, when Fe-doped InP is crystal-grown such thatthe crystal grown in the (100) direction is discouraged as in theprevious embodiments, InP is buried between the side surfacesperpendicular to the (011) direction of the optical elements 181 asshown in FIGS. 19B and 19E. The result is that the spacing in the(0{overscore (1)}1) direction is narrowed.

[0240] Consequently, a buried layer 182 b is formed flat as illustratedin FIGS. 19C and 19F. Note that in FIGS. 19A to 19F, the same referencenumerals as in FIGS. 1A and 1B denote the same parts.

[0241] Eleventh Embodiment

[0242] In the above 10th embodiment, the planar shape of the opticalelement is square. However, the present invention is not restricted tothis.

[0243] Even when optical elements 201 whose planar shape is circular arearranged into an array as illustrated in FIG. 20C, the same effect as inthe above embodiment can be attained by arranging a dummy portion 202having the same shape at the center of a square at the four corners ofwhich four optical elements 201 are arranged.

[0244] In this structure, InP 203 to be buried so grows as to spreadinto an elliptical shape as shown in FIG. 20A and FIGS. 34A and 34B.FIG. 34A is a perspective view of a circular optical element, and FIG.34B is a plan view showing the state in which InP grows around theoptical element.

[0245] As illustrated in FIG. 20B, immediately before completion of theburying, the InP 203 so crystal-grows as to leave gaps 204 behind.Finally, a flat buried layer 6 is formed as in FIG. 20C.

[0246] To apply a driving current (voltage) to the optical semiconductordevice as described in the first embodiment, it is necessary to form anelectrode with a predetermined area to be connected to the electrodecontacting layer 5 (FIGS. 1A and 1B).

[0247] When, however, the trenches 14 and 15 are formed and aninsulating or semi-insulating semiconductor such as Fe-doped InP isburied in these trenches as illustrated in FIGS. 1A and 1B, most regionof the electrode is formed on the electrode contacting layer via a thininsulating layer.

[0248] Since this produces a capacitance, the capacitance of a wholeelement cannot be satisfactorily decreased. This makes a high-speedoperation impossible.

[0249] This parasitic capacitance can be decreased by forming a trenchin the region below the electrode formation portion and burying thetrench with, e.g., Fe-doped InP.

[0250] Optical semiconductor devices according to other embodiments ofthe present invention in which a trench is also formed in an electrodeformation region will be described below.

[0251] Twelfth Embodiment

[0252]FIG. 21 is a perspective view showing the arrangement of a portionof an optical semiconductor device according to the 12th embodiment ofthe present invention.

[0253] Referring to FIG. 21, a substrate 210 is made from InP whoseorientation is (001), and a laser structure L consists of a materialsystem of InP and InGaAsP. This laser structure L is formed bysandwiching a laser active layer 211 between two layers different inconductivity type.

[0254] On this laser structure L, a laser contact layer 212 and asemiconductor protective layer 213 are stacked.

[0255] In FIG. 21, reference numeral 214 denotes a mask consisting ofSiO₂ in which holes are formed in regions corresponding to trenches 216and 217 to be buried.

[0256] By using this mask 214, etching is performed to a depth reachingthe substrate 210 by reactive ion etching (RIE) using C₂H₆ and H₂ asreactive gases. Consequently, the trenches 216 about 10μm wide areformed, and a mesa structure 215 which oscillates a laser is formedaccordingly.

[0257] In this device, the extending direction of the mesa structure 215is the (011) direction, and the longitudinal direction of the trenches217 in a region on which an electrode is formed is the (0{overscore(1)}1) direction.

[0258] Since the growth occurs faster in the (011) direction aspreviously described, the trenches 217 are formed to have a rectangularplanar shape of 50m×100 μm which is long in the (011) direction.

[0259] After the trenches 216 and 217 are thus formed, the mask 214 isused as a selective growth mask to bury the trenches 216 and 217 withsemi-insulating InP using Fe as a dopant in accordance with HVPE.

[0260] Thereafter, the mask 214 and the semiconductor protective layer213 are removed. As illustrated in FIG. 22, an insulating film 219 is soformed as to cover a portion up to the top of a buried layer 218, and anohmic electrode 221 is formed on the mesa structure 215 through anopening 220.

[0261] An electrode 222 is then formed in a region on the trenches 217so as to be connected to the ohmic contact 221.

[0262] Consequently, the trenches 216 and 217 are buried flat, and theparasitic capacitance of the electrode 222 can also be decreased.

[0263] Note that in FIG. 22, in regions indicated by the dotted lines,the trenches 216 and 217 buried with the buried layer 218 made from theFe-doped semi-insulating InP are present below the insulating film 219.

[0264] As described above, even when the trenches 216 and 217 are buriedto a depth of about 4 μm or more, a flat burying is readily possible asin the previous embodiments. This can realize a low-capacitance elementeven after the electrode 222 is formed on the trenches 217.

[0265] As an example, when the depth of the trenches 216 and 217 isabout 10 μm and the dimensions of the electrode 222 are 100 μm×100 μm,the capacitance in the formation region of the electrode 222 isapproximately 0.1 pF, and the 3-dB band is 50 GHz in the case of 50-Ωtermination.

[0266] In contrast, if the electrode 222 is formed without forming thetrenches 216 and 217, the capacitance in the electrode formation regionis approximately 0.6 pF, and the 3-dB band is 8 GHz in the 50-Ωtermination.

[0267]FIG. 23 shows a trench structure when the mesa structure 215points in the (0{overscore (1)}1) direction.

[0268] Referring to FIG. 23, the width of the trenches 216 is about 50μm, i.e., the same as the width of the trenches 217 in the electrodeformation region. The rest of the structure and the device formationprocess are identical with those of the device shown in FIG. 21.

[0269] Thirteenth Embodiment

[0270]FIG. 24 is a perspective view of the arrangement of still anotherembodiment of the semiconductor device according to the presentinvention, which illustrates a transverse light injection type bistablelaser. In FIG. 24, the same reference numerals as in FIG. 23 denote thesame parts.

[0271] The transverse light injection type bistable laser shown in FIG.24 is formed by the same fabrication steps as in the 12th embodimentdescribed above.

[0272] The semiconductor device thus formed has a laser unit 241, asaturable absorbing unit 242, and an optical amplifier 243.

[0273] Note that in FIG. 24, in regions indicated by the dotted lines,trenches 216 and 217 buried with a buried layer 218 are present below aninsulating film 219.

[0274] In the transverse light injection type bistable laser with theabove arrangement, an epitaxial wafer has a quantum well structure, anda bistable operation is performed due to the interaction between thelaser unit 241 and the saturable absorbing unit 242.

[0275] The saturable absorbing unit 242 applies a voltage lower than thbuilt-in voltage of a laser diode by using the supersaturationcharacteristics of a quantum well.

[0276] Control light is amplified by the optical amplifier 243 and fedinto the saturable absorbing unit 242.

[0277] Once the control light is incident, the laser unit 241 which hasnot performed laser oscillation starts oscillating and keeps emittingthe laser light even after the control light disappeared (bistableoperation).

[0278] To stop the laser light once oscillated, a deep reverse biasvoltage must be applied to the saturable absorbing unit 242.

[0279] In the semiconductor device illustrated in FIG. 24, alow-capacitance electrode 244 and a low-capacitance interconnect line245 according to this embodiment are formed to rapidly apply a deepreverse bias to the supersaturation absorbing unit 242.

[0280] Fourteenth Embodiment

[0281]FIG. 25 is a perspective view of the arrangement of still anotherembodiment of the semiconductor device according to the presentinvention, which shows a semiconductor integrated mode-locked laser. InFIG. 25, the same reference numerals as in FIG. 24 denote the sameparts.

[0282] Referring to FIG. 25, this semiconductor integrated mode-lockedlaser has a laser unit 241 and an optical modulator 251.

[0283] Note that in FIG. 25, in regions indicated by the dotted lines,trenches 216 and 217 buried with a buried layer 218 are present below aninsulating film 219.

[0284] Note also that an epitaxial wafer consists of an active layer 211having quantum wells for laser oscillation and a light absorbing layer252 with a quantum well structure for the optical modulator 251.

[0285] The active layer 211 is removed from an optical modulation region253.

[0286] This epitaxial structure is fabricated as follows. That is,layers up to the active layer 211 are formed on a substrate 210, and theoptical modulation region 253 of this active layer is etched away.

[0287] Subsequently, a cladding layer (not shown), a contact layer 211,and an InP protective layer (not shown) are regrown on the entiresurface, thereby performing the regrowth on the entire epitaxial wafer.

[0288] Mode-locked light pulses can be obtained by modulating theoptical modulator 251 at a period in which light travels around once inthe element.

[0289] Since this period is usually shorter than 100 ps, an electrode255 and an interconnect line 254 to which a modulated signal is appliedmust have low capacitances.

[0290] As illustrated in FIG. 25, therefore, the low-capacitanceelectrode 255 and the low-capacitance interconnect line 254 according tothis embodiment are formed.

[0291] The trenches formed below the electrode 255 and the interconnectline 254 and buried by the buried layer 218 are as deep as approximately10 μm to obtain a high speed of a period of 100 ps or more.

[0292] Fifteenth Embodiment

[0293]FIG. 26 is a perspective view of the arrangement of still anotherembodiment of the semiconductor device according to the presentinvention, in which a waveguide photodetector is illustrated.

[0294] The device shown in FIG. 26 is different from the device shown inFIG. 22 in that a light absorbing layer 261 is formed in place of theactive layer 211 in FIG. 22 and a wide photodetector waveguide 262 forimproving optical coupling and an electrode 263 having a deep trench forimproving the operating speed also are formed.

[0295] In this arrangement, incident light from a device end face 264applies a reverse bias voltage to the device through the electrode 263with a low capacitance and at the same time extracts a detection signalto the outside.

[0296] Note that in FIG. 26, in regions indicated by the dotted lines,trenches buried with a buried layer 218 are present below an insulatingfilm 219.

[0297] In the embodiments previously described, the insulating film 219is formed on the trenches 217 for forming the electrode 222, as, forexample, illustrated in FIG. 22.

[0298] Since, however, the trenches 217 are buried with thesemi-insulating buried layer 218, the insulating film 219 is notnecessarily formed.

[0299] If this is the case, as illustrated in FIG. 27, an insulatingfilm 219′ can be formed only on top of laser regions M left behind.

[0300] Note that the insulating film 219 is apparently sometimesunnecessary even when the buried layer 218 is not entirelysemi-insulating (e.g., when a layer having a different composition fromthat of the buried layer 218 is inserted into the layer 218 and thislayer is an undoped n-type conductive layer).

[0301] Also, as shown in the sectional view of FIG. 28, it is onlynecessary to form the electrode 222 so as not to cover laser regions Mleft behind.

[0302] The mask 214 shown in FIGS. 21 and 23 can also be directly usedas the insulating film 219′ on the remaining laser regions M.

[0303] Note that it is also possible to form interconnections forexternal wiring with the electrode 222 not formed on the laser regionsM.

[0304] As shown in FIG. 28, even if a metal wire or metal ribbon 281 forexternal wiring is bonded to an electrode formation portion 217′, gaps282 are formed on the laser regions M.

[0305] Accordingly, the upper portions of the laser regions M do notdirectly contact the metal wire or metal ribbon 281.

[0306] In the above embodiments, the electrode 222 and the interconnectline are formed on the laser structure L via the insulating film 219.However, the present invention is not limited to these.

[0307] That is, the same effect as above can be attained so long as atleast the insulating film 219 is formed on the laser region M leftbehind between the two trenches 217 and on the laser region M′ (see FIG.21) left behind between the trenches 217 and the adjacent trench 216.

[0308] Also, in a structure in which the trenches 217 and 216 areconnected, the insulating 219 need not be used.

[0309] In the above embodiments, hydride vapor phase epitaxy (HVPE) isused as the crystal growth method. However, the present invention isalso effective when other crystal growth methods are used, becausetrenches can be buried by other crystal growth methods.

[0310] Particularly, suitable methods other than HVPE are chloride vaporphase epitaxy (CVPE) and metal organic vapor phase epitaxy (MOVPE).

[0311] In the above embodiments, iron-doped InP is used to bury thelaser mesa formation region and the electrode formation region. However,InP doped with titanium or chromium, rather than iron, or InP doped withboth titanium or chromium and iron, is also a high-resistancesemiconductor. Therefore, a semiconductor layer made from such amaterial can also be used.

[0312] It is also possible to insert at least one semiconductor layerhaving different composition, such as InGaAsP, between thesehigh-resistance semiconductor layers.

[0313] This semiconductor layer functions as a block layer for carriersinjected from the laser region into the high-resistance semiconductorand not captured by an impurity such as iron.

[0314] The semiconductor layer having this function is thereforeeffective to obtain a high laser output (if two or more such layers areto be inserted, these inserted layers can be different in composition).

[0315] Furthermore, an electrode formation r gion can be buried even ina multilayered structure including this inserted layer.

[0316] Although the above embodiments have been described by taking anInP laser as an example, the present invention can be practiced by usingsome other material system.

[0317] As an example, the present invention can be practiced by asemiconductor laser in which a cladding layer and a high-resistanceburied layer are made from GaAs and an active layer consists of strainedquantum wells having an InGaAs well layer.

[0318] To increase the resistance of GaAs of a buried layer, it is onlynecessary to dope vanadium, chromium, or oxygen.

[0319] The resistance can also be increased by optimizing the growthconditions and decreasing the background carrier concentration.

[0320] In forming a block layer in the GaAs buried layer, strainedInGaAs can be used as the block layer.

[0321] The present invention can be similarly practiced by a GaAs/AlGaAslaser. In this laser, a buried layer is AlGaAs, and an inserted layer isGaAs or AlGaAs having a different composition from that of the buriedlayer.

[0322] Oxygen can be doped to increase the resistance of AlGaAs of theburied layer.

[0323] The resistance can also b increased by optimizing the growthconditions and decreasing the background carrier concentration.

[0324] Moreover, the present invention can be practiced by using variousother material systems.

[0325] According to the present invention as has been described above,in burying an optical element including an active layer with, e.g., asemi-insulating semiconductor buried layer, a semiconductor regionhaving side surfaces opposing the side surfaces of the optical elementis formed, thereby forming walls such that in a portion where the growthrate of crystal growth is lower, the distance between the side surfaceof the optical element and the side surface of the semiconductor regionis shorter. Portions between these side surfaces are buried with theburied layer.

[0326] Consequently, even if the optical element is constituted bywaveguides in different directions, a buried heterostructure (BHstructure) which is buried flat can be effectively formed.

[0327] Also, when a plurality of optical elements are formed into anarray, a dummy portion is formed at the center of a square at the fourcorners of which the optical elements are arranged. Portions betweenthese optical elements and the dummy portion are buried with a buriedlayer.

[0328] Accordingly, even when an optical element is a rectangular orcircular surface emission laser and a plurality of such optical elementsare arranged, a flatly buried heterostructure can be formed.

What is claimed is:
 1. A method of fabricating an optical semiconductordevice, comprising the steps of: forming a plurality of opticalsemiconductor elements on a semiconductor substrate; forming a dummyportion in a center of a square at four corners of which said opticalsemiconductor elements are arranged; and forming a buried layer by vaporphase epitaxy so as to bury a portion between said optical semiconductorelements and said dummy portion.
 2. A method according to claim 1,wherein said buried layer is formed by vapor phase epitaxy using one ofa chloride-based source gas and a hydride-based source gas.